In large-capacity electric power conversion devices, the converter output is high voltage or large current, and therefore, many large-capacity electric power conversion devices are configured with a plurality of converters multiplexed in series or parallel. Multiplexing converters enables not only increase in the converter capacity, but also reduction in harmonics contained in an output voltage waveform by synthesizing outputs. As a result, harmonic current flowing to a grid can be reduced.
As means for multiplexing converters, there is known a modular multilevel converter in which outputs of a plurality of converters are connected in cascade. Each arm of the modular multilevel converter is composed of a plurality of converter cells connected in cascade.
In a conventional modular multilevel converter, each of a first arm and a second arm for each phase, which are provided between an AC terminal for each phase and positive and negative DC terminals, has a chopper cell (converter cell) and a reactor. The chopper cell has two semiconductor switching elements connected in series to each other, and a DC capacitor connected in parallel thereto. In each of the first arm and the second arm, the same number of chopper cells are connected in series via their respective output ends. Using voltage commands for the first arm and the second arm, semiconductor switching elements in the chopper cells in the first arm and the second arm are ON/OFF controlled, thereby generating AC voltage at the AC terminal and DC voltage at the DC terminals (see, for example, Non-Patent Document 1).
Another modular multilevel converter is disclosed which uses a multilevel circuit in which a converter cell has semiconductor switching elements in a full-bridge form, for the purpose of suppressing short-circuit current that occurs when DC terminals P, N are short-circuited (see, for example, Patent Document 1).
Further, a Static Synchronous Compensator (STATCOM) using a modular multilevel converter of star-connection type in which AC sides of converter cells are connected in series for each phase, is proposed (see, for example, Non-Patent Document 2).